In order to obtain a copied image, an image forming process of an electrophotographical apparatus, such as a copying machine, generally begins with pressing a copying (image forming) start key and then follows a preprogrammed sequence: namely, driving a photosensitive body, charging the photosensitive body, forming a latent image by exposure, developing the image, feeding a sheet, transferring the image to the sheet, fixing the image to the sheet, cleaning the photosensitive body surface and eliminating residual potential of the photosensitive body.
In recent years, as there are market demands for improving copied image quality, image-forming process factors, such as (1) charged potential of the photosensitive body, (2) optical lamp voltage and (3) toner density, are controlled in order to obtain solid black and half-tone images of uniform density and high quality.
Here are some specific examples of factor controlling methods: (1) The charged potential is controlled by adjusting voltage applied by a charger on the basis of a difference between the charged potential measured by a surface electrometer and reference potential. (2) The optical lamp voltage is controlled by adjusting light source lamp voltage on the basis of a difference between post-exposure surface potential measured by a surface electrometer and reference potential. (3) The toner density is controlled by forming a toner image of uniform density patch on part of the photosensitive body and adjusting the toner-to-developer ratio on the basis of density of the toner image measured by an optical sensor.
In order to obtain a high quality copied image, each of these controlling methods controls a process factor by firstly forming an electrostatic latent image or a toner image on part of a photosensitive body surface, then measuring surface potential of the electrostatic latent image or the toner image, or density of the toner image, and finally using control information outputted by a controlling circuit based on information obtained through the measurement. The controlling methods are normally carried out before document copying.
The electrostatic latent image is, for example, formed on the photosensitive body surface by exposing the uniform density patch provided on part of a document platen of a copying machine after a predetermined time elapses since a copying instruction through a copying start key. The toner image is, for example, formed on the photosensitive body surface by developing the exposed uniform density patch with toner.
But, the photosensitive body is not always in the same rotation-starting position upon receiving the copying instruction through the copying start key. As the rotation-starting position of the photosensitive body changes, the electrostatic latent image of the patch or the toner image is not always formed at the same place on the photosensitive body surface.
Consequently, even if uniform density patch is used, various irregularities, depending on the forming place of the electrostatic latent image of the patch, can cause fluctuation in measurement of the surface potential obtained from the electrostatic latent image of the patch, or fluctuation in measurement of the toner image density obtained from the toner image. Here the measurement fluctuation of the toner image density means fluctuation in output by an optical sensor. The irregularities here include mechanical irregularities, such as configuration (roundness) irregularity and rotational displacement of the photosensitive body, and varying photosensitivity depending on places on the photosensitive body surface. Therefore, control information for obtaining the high quality image varies every time a copying process is carried out and here occurs a problem that the best copied image is not constantly available.
Accordingly in recent years, in order to solve the problem, a marking area is provided on the photosensitive body which serves as a reference place. The marking area makes it possible to always measure the surface potential of the exposed electrostatic latent image with respect to the uniform density patch, and the toner image density etc. at the same place on the photosensitive body every time the copying process is carried out. Photosensitive bodies for electrophotographical use having this kind of marking area, for example, are disclosed in Japanese Laid-Open Patent Applications No. 6-35379/1994 (Tokukaihei 6-35379) and No. 6-149136/1994 (Tokukaihei 6-149136). Disclosed in Japanese Laid-Open Patent Application No. 6-35379/1994 is a photosensitive body for electrophotographical use having a marking area provided by, for example, a grinding stone, a grinding processing tape or grinding agent. Disclosed in Japanese Laid-Open Patent Application No. 6-149136/1994 is a photosensitive body for electrophotographical use having a marking area provided through a grinding processing by a laser beam.
Photosensitive bodies made of organic photoconductive material are widely used for the above photosensitive bodies, because the material has beneficial characteristics such as non-polluting and easy in providing a film and in manufacturing. Especially, photosensitive bodies on which a charge producing layer and a charge transporting layer are laminated (so-called lamination type photosensitive bodies) are most widely used.
But, lamination type photosensitive bodies which have been commercialized so far have problems of electrical characteristics such as (1) insufficient photosensitivity, (2) high residual potential of the photosensitive body surface and (3) slow light response. In order to solve the problems, the photosensitivity is improved by thickening the photosensitive layer but here occurs another problem: a thick photosensitivity layer (30 .mu.m to 40 .mu.m thick) reduces outline clarity and clearness of the copied image.
Meanwhile, in order to improve the clarity and the clearness of the copied letters, a photosensitive body having a photosensitive layer less than 25 .mu.m thick has been developed through an improvement in photosensitive material and photosensitive layer structure. But, when the marking area for controlling process factors is provided on this kind of thin photosensitive body, a surface of the photosensitive layer provided right on the marking area becomes less smooth. Here occurs other problems such as inadequate cleaning and toner falling.